bridge: simplify the flush_store by calling store_bridge_parm

There are some repetitive codes in flush_store, we can remove
them by calling store_bridge_parm, also, it would send rtnl notification
after we add it in store_bridge_parm in the following patches.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>